<?php
/**
 * Created by PhpStorm.
 * User: User
 * Date: 2017/10/16
 * Time: 19:13
 */
class Printer
{
    private static $obj = null;

    private function __construct()
    {

    }

    public static function getInstance()
    {
        if (is_null(self::$obj)) {
            self::$obj = new Printer();
        }
        return self::$obj;
    }

}

$p1=Printer::getInstance();
$p2=Printer::getInstance();
var_dump($p1);
var_dump($p2);
var_dump($p2==$p1);